In the prior art, a zapping operation on an IP decoder follows the various steps shown in the time chart of FIG. 1. In this figure, the initial time (t=0) is marked as being the time when a user performs an action 100 to carry out a change of service. Such an action is most often constituted by pressing on one or more appropriate keys of a remote control associated with the decoder. The passage of zapping time is marked with relation to this time reference.
A first step 101 then resides in the software for operating the IP decoder to take the action of the user into account. The duration of this step is typically 150 ms (milliseconds). This duration is particularly utilized so that the operation software releases a specific application for zapping, possibly for an exchange with a remote server to obtain various requests, and also to verify that the user does not, during this time interval, send an instruction cancelling the zapping command initially received.
Then a second step 102 follows, in the course of which the IP decoder executes a command known as the “IGMP join command.” The duration of this step is close to 400 ms; it corresponds to a decoder connection time, the time necessary, from the execution of the IGMP join command, to receive the first appropriate data packets extracted from a multicast signal stream. In the case of terrestrial, satellite or cable decoders, the decoder connection time corresponds to the time that elapses between an action on the decoder tuner and the effective receipt of the first data packets.
An operation to store in a buffer memory the data received in the stream, with demultiplexing of various components, is then carried out. However, such an operation takes an insignificant amount of time due to the hardware of the various intervening components.
Next the acquisition of various tables and parameters necessary to perform the zapping operation is carried out:
Thus, the next step 103, that typically lasts 75 ms, is a PAT table acquisition step.
Then, step 104 follows, with an average duration of 75 ms, in which the PMT table is acquired.
Next, in the following step 105, the access control is carried out; this lasts approximately 300 ms and essentially consists of:—an analysis of the descriptors from the PMT table (mainly the descriptor identified CA_descriptor—tag: 0x09);—then placing filters at the level of a demultiplexer of the decoder to receive the ECM (Entitlement Control Message, defined in the standard ISO/IEC 13818-1);—then a dialog with the smart card of the decoder to define if the user has rights over the desired service;—if yes, descrambling the ECM into control words CW; The control words will then be provided to the descrambler to access in unscrambled mode the various components of the desired service.
Once the various parameters and tables are acquired, in the next step 106, the audio and video decoding operation is carried out for the desired service. The time taken between sending the audio and video data to the audio and video decoder of the IP decoder and the effective display on the display screen is on the order of 1500 ms.
Thus, to carry out a zapping operation in IP decoders of the prior art, a duration close to 2500 ms is required.
The stream of signals transmitted to the IP decoders passes through DSLAM type equipment. The stream is then directly sent to the various IP decoders, with a progressive attenuation of the useful signal. The farther that an IP decoder user is situated from the DSLAM that provides the signal stream, the higher the signal-to-noise ratio, and the higher the risk of error in the reconstruction of audio-video signals. Consequently, in order to increase the number of users likely to be accessible from a given DSLAM without the signal being prohibitively noisy, the utilization of an advanced protection system against errors likely to be produced during data transmission has recently been proposed.
The FEC (Forward Error Correction) system has been chosen. In the future, other systems may possibly be chosen, without necessarily challenging the operation of the method according to the invention. In the implementation of the FEC system, the transmitter adds redundancy in order to enable the recipient, in this case the IP decoder, to detect and correct part of the errors that could have been produced. In addition, such a system, even if it necessitates information redundancy, enables retransmission to be avoided, and thus enables bandwidth to be saved.
In general, the operation of the FEC system may be considered as follows: The signal stream received by the IP decoder is constituted of a plurality of multiplexed components. In particular, the FEC system distributes the audio and video components into various sets of data, at least a first set of data and a second set of data, that are each organized in matrix form, respectively a first matrix and a second matrix of audio and video data. A redundant correction datum corresponds to each line or column of the matrix: If one of the audio-video data from a column was tainted with an error, then the datum under consideration may be corrected from the correction datum associated with the column under consideration, and non-corrupted data from the same column.
FIG. 2 represents an example of embodiment of a zapping operation on a signal stream in which the FEC correction system is in place. The first two steps of the zapping operation from this example are identical to the two steps from the example illustrated in FIG. 1. At the end of step 102, marked by a connection time on the order of 400 ms, in step 201, at least a first data set and a second data set are stored in buffer memory and then the audio-video components thus stored are corrected.
In the FEC system, correction data from the first data set are transmitted with data from the second data set. Thus, non-correctable error risks are limited by spacing in time the audio-video data and the correction data likely to be used to correct the audio-video data under consideration. A consideration of such an operation is that it is necessary to store at least two matrices, or two data sets, before being able to initiate FEC system correction operations. Thus, by considering a signal stream of the SD (Simple Definition) type at 2.2 Mbits/s, and a matrix size of 8 columns by 5 lines, the buffer memory storage time and the correction implementation time is close to 400 ms.
Step 201 thus ends, with the corrected audio-video components obtained, approximately 950 ms after the action by the user to change the service. Step 201 is crucial to offering good audio-video quality since, contrary to the operating components, any loss of audio-video data risks creating visual defects, of the macro block, frozen or jerky picture, black screen, etc., type. At the end of step 201, steps 103, 104, 105 and 106, detailed during the description of the first example of embodiment, are repeated.
In such an example, the zapping operation is thus on the order of 2900 ms; Thus a significant increase on the order of 20% of the zapping time is observed.